1. The Field of the Invention
The present invention relates to the formation of a film substantially composed of a nitride of a diffusion barrier material situated on a semiconductor substrate. More particularly, the present invention is directed to the formation of a large grain diffusion barrier material nitride film situated on a semiconductor substrate in a process suitable for forming a diffusion barrier and for forming a refractory metal salicide stack structure with a diffusion barrier material nitride cover layer.
2. The Relevant Technology
In the manufacturing of an integrated circuits upon a semiconductor substrate, barriers are often needed to prevent the diffusion of one material to an adjacent material. For instance, when aluminum contacts silicon surfaces, spiking can occur, and when aluminum comes into direct contact with tungsten, a highly resistive alloy is formed. Diffusion barriers are structures commonly used to prevent such undesirable reactions.
In the context of this document, the term xe2x80x9csemiconductor substratexe2x80x9d is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term xe2x80x9csubstratexe2x80x9d refers to any supporting structure including but not limited to the semiconductor substrates described above.
Titanium nitride has formerly been the material of choice for forming diffusion barriers and adhesive layers where conductive interfaces must be maintained. More recently, however, tungsten nitride has begun to appear more favorable and is in many applications replacing titanium nitride. Tungsten nitride has advantages over titanium nitride in that it has a lower resistivity and is thus more suitable for use in conductive interfaces in high speed applications. It is also more thermally stable, making it more suitable for the high temperature processing common in integrated circuit manufacturing.
One conventional method of forming tungsten nitride diffusion barriers is with chemical vapor deposition (CVD). Conventional chemical vapor deposition processes react tungsten with gaseous nitrogen at a high temperature in atmosphere of fluorine to form a film of tungsten nitride. Problems attendant to this process include the detrimental tendency of the fluorine to attack exposed surfaces of semiconductor substrates on which the diffusion barrier is being formed. The lack of cleanliness of chemical vapor deposition processes also presents problems. Consequently, the art has looked to other methods of depositing tungsten nitride films.
Physical vapor deposition (PVD) is another convention method of forming tungsten nitride diffusion barriers and is an alternative to the use of chemical vapor deposition for depositing tungsten nitride. The conventional physical vapor deposition technology involves reactive sputtering from a tungsten target in an atmosphere of gaseous nitrogen with an argon carrier gas. In this conventional saturated reactive sputtering mode, the volume ratio of nitrogen (N2) to the argon carrier gas is selected such that the tungsten target is filly nitrided by surface dissociated nitrogen.
This type of conventional PVD process is highly reactive and causes simultaneous high density, nonuniform nucleation and grain growth, and results in a highly columnar, small grain film with a high resistivity. The small grain size, when the grains come into contact with adjacent layers such as aluminum which is of a large grain size, tends to cause stress at the interface between the layers and can cause the layers to peel away from each other. Also, the high amount and irregularity of the grains formed by the conventional process tend to cause voids, which give rise to electromigration and consequently, reduced diffusion barrier abilities. Voids are especially prone to forming at interfaces between adjoining layers.
A further problem with the conventional physical vapor deposition process is a columnar structure that is exhibited by the resulting film. The columnar structure, which appears as spikes between the grains, serves as a channel for diffusion and reduces the effectiveness of the columnar structure as a diffusion barrier.
One application for tungsten nitride films is the formation of diffusion barriers between the tungsten of tungsten plugs and adjoining metallization layers on the surface of the wafer. Such a diffusion barrier is shown in FIG. 1. Therein is shown a tungsten plug 14 extending down to a silicon substrate 10 with an overlying metallization layer 16 and an intervening diffusion barrier 12. The tungsten plug structure is one example of an application where tungsten nitride has been found as a suitable replacement for titanium nitride, as it is easily formed over the tungsten plug. Nevertheless, void formation and interfacial stress inherent to the conventional physical vapor deposition processes, along with the aforementioned problems associated with fluorine processing and cleanliness for chemical vapor deposition processes are detriments to the use of tungsten nitride for such applications.
A further application where an improved method for forming tungsten nitride films could be favorably used is in the formation of low resistivity tungsten nitride/titanium silicide stack. A titanium silicide (TiSi2) self aligned diffusion barrier, known as titanium salicide, is formed by sputtering titanium on a polysilicon and annealing the deposited titanium at 650xc2x0 C. after masking in a gaseous nitrogen environment to form titanium salicide in a C-49 phase. A second anneal at 850xc2x0 C. transforms the titanium salicide to a more thermally stable C-54 phase and is followed by a standard wet strip. Titanium salicide stacks are commonly used for forming word and bit lines in DRAM memory structures and for forming local interconnects to CMOS gate structures.
The problems exhibited by conventional titanium silicide structures include agglomeration at the titanium silicide and polysilicon interface and decomposition of the titanium silicide back into titanium and silicon at high temperatures that results in high resistivity.
A need exists for a process of forming a film substantially composed of a nitride of a diffusion barrier material which overcomes the problems existing with conventional chemical vapor deposition and physical vapor deposition processes, and which can be used to form a suitable diffusion barrier that has low resistivity, large grain size, low interfacial stress, and which is thermally stable. Such a process would be beneficial if it were compatible with and solved the aforementioned problems existent with processes for forming the titanium salicide stack structure.
The present invention seeks to resolve the above and other problems that have been experienced in the art. More particularly, the present invention constitutes an advancement in the art by providing a process for forming a film that is substantially composed of a nitride of a diffusion barrier material. The diffusion barrier material is substantially composed of a material selected from a group consisting of tungsten alloys of Group III and Group IV early transition metals and molybdenum alloys of Group III and Group IV early transition metals. The present invention provides a process for forming the nitride of the diffusion barrier material having a grain size in a range from about 1000 Angstroms to about 2000 Angstroms, where the diffusion barrier material nitride film has a substantially crystalline structure and a peak-to-valley roughness of less than about ten percent of the thickness thereof.
In accordance with the invention as embodied and broadly described herein in the preferred embodiment, a process is provided for manufacturing a nitride of a diffusion barrier material with physical vapor deposition (PVD) which results in a large grain, low stress film. Also provided are applications of the nitride of a diffusion barrier material such as a cover layer for a refractory metal salicide stack, where the refractory metal salicide stack is nitrided.
The inventive process for forming the film composed of a nitride of a diffusion barrier material includes providing a surface layer located on a semiconductor substrate. A layer of a diffusion barrier material is sputtered on the surface layer in an environment comprising a gaseous nitrogen content. The gaseous nitrogen content is selected such that a nucleation of a nitride nuclei of the diffusion barrier material is incorporated in the layer of the diffusion barrier material. There will preferably be between about 4xc3x97108 to about 4xc3x97105 nitride nuclei of the diffusion barrier material per cm of the diffusion barrier material.
After the diffusion barrier material is deposited, grains are grown of a nitride of the diffusion barrier material in the layer of the diffusion barrier material in an environment containing nitrogen to form a layer of a nitride of the diffusion barrier material.
In another application, a contact plug is formed having an end upon an active area in a semiconductor substrate and an opposite end with a surface layer thereon. A layer of a diffusion barrier material is sputtered on the surface layer in an environment comprising a gaseous nitrogen content. The gaseous nitrogen content is selected such that a nucleation of a nitride nuclei of the diffusion barrier material is incorporated in the layer of the diffusion barrier material, wherein there is between about 4xc3x97108 to about 4xc3x971015 nitride nuclei of the diffusion barrier material per cm2 of the diffusion barrier material. The diffusion barrier material is substantially composed of a material selected from a group consisting of tungsten alloys of Group III and Group IV early transition metals and Mo alloys of Group III and Group IV early transition metals.
After the nitride nuclei of the diffusion barrier material are formed, grains are grown of a nitride of the diffusion barrier material in the layer of the diffusion barrier material in an environment containing nitrogen to form a layer of a nitride of the diffusion barrier material. Next, a metallization layer is formed upon the layer of the nitride of the diffusion barrier material.
The general process of the present invention for manufacturing film substantially composed of a nitride of a diffusion barrier material comprises in a first step, producing an underlying surface layer upon which the diffusion barrier material film is to be formed. The underlying surface layer may comprise refractory metal, as when forming a diffusion barrier between a refractory metal plug and an overlying aluminum interconnect line, or the underlying layer may be doped silicon, polysilicon, titanium, or any other suitable material for semiconductor applications.
In a further step, the diffusion barrier material is deposited on the underlying layer using unsaturated physical vapor deposition reactive sputtering in a nitrogen environment so that a nitride of the diffusion barrier material results. This is typically conducted in a physical vapor deposition chamber with parameters known in the art.
In one embodiment, a gaseous mixture of nitrogen in an argon carrier is selected and passed into the PVD chamber. The gaseous mixture is selected to have an optimum nitrogen content level that causes a light nucleation of the nitride of the diffusion barrier material and which results in no refractory metal nitride grain growth. A light, highly uniform nucleation of the nitride of the diffusion barrier material results from the low nitrogen content which is uniformly distributed to serve as nuclei for later grain growth. Thus, the PVD process is used as a uniform nucleation process but not a grain growth process.
The optimum nitrogen content level is the nitrogen/argon gas mixture that causes surfaces adjacent to the target such as side shields to be substantially coated with nitrogen from the nitrogen environment, but cause the target to be only lightly coated.
Examples of mixtures for forming the nitrogen environment include, for example, diatomic nitrogen in a plasma driven process, diatomic nitride, and nitrous oxide in an argon carrier gas.
In order to determine the optimum nitrogen content operating level, the following steps are performed. First, using the physical vapor deposition chamber and operating parameters that will be used during the deposition, the diffusion barrier material deposition rate is experimentally calculated as a function of the nitrogen content in the physical vapor deposition chamber environment and plotted, with the diffusion barrier material deposition rate being plotted on one axis and the nitrogen content from 0 to approximately 100% volume ratio in argon being plotted on the second axis. The resulting plot will have a curve that starts out with a slight taper, drops sharply, and then tapers out and substantially levels off again at the bottom. This curve will occur at different levels of nitrogen content and differing diffusion barrier material deposition rate levels, but the characteristic slope of the curve with the sharp drop and leveling off at the bottom has been found to be consistent at different power levels and operating parameters and within various chambers.
The nitrogen content operating level is selected to correspond to just prior to the point on the plot of the steepest deposition rate slope in the downward direction, which also corresponds approximately to the point of maximum rate of change of the slope in the downward direction. Once the nitrogen content operating level has been selected, the diffusion barrier material deposition is conducted within the physical vapor deposition chamber to form a diffusion barrier material film with lightly nucleated and uniformly distributed nitride seeding of the diffusion barrier material.
The next step is to grow a near epitaxial quality crystalline diffusion barrier material nitride structure from the diffusion barrier material film. This is known as xe2x80x9cgrain growth.xe2x80x9d The grain growth step is conducted by heating the diffusion barrier material film in a nitrogen environment, typically to a temperature of between about 600xc2x0 C. and 700xc2x0 C., and using a rapid thermal nitridization process.
The resulting diffusion barrier material nitride film exhibits large grain structure that will result in low stress to adjoining layers, a high surface smoothness, and a high thermal stability, thus providing the capability of serving as an improved diffusion barrier.
One application of the diffusion barrier material nitride film of the present invention is to form a cover layer to a salicide layer of a refractory metal, such as titanium. When so doing, the first step is to form an underlying layer on which to form the refractory metal salicide. The underlying layer is typically polysilicon, though it could also comprise doped silicon, or other suitable layers. The underlying layer is lightly nitrided in an ion implantation tool.
Subsequently, a refractory metal film, such as titanium, is deposited over the underlying layer using physical vapor deposition in an environment of light nitridization. The light nitridization typically comprises a volume of nitrogen of up to about 3% in an inert carrier gas such as argon.
In a further step, the lightly nitrided refractory metal film is annealed in an atmosphere of nitrogen to form a nitride of the refractory metal. In a typical process, this comprises a first anneal at about 650xc2x0 C. followed by a stripping of unreacted refractory metal remaining on the surface, and a second anneal conducted at approximately 850xc2x0 C., which transforms a silicide of the refractory metal to a less resistive and more thermally stable phase species.
Diffusion barrier material is then deposited in accordance with the process for manufacturing a diffusion barrier material film as described above, and is grown into a nitride of the diffusion barrier material, also in accordance with the process as described above.
The resulting lightly nitrided refractory metal salicide structure exhibits beneficial qualities for use in structures such as word and bit lines and interconnect access lines, with a reduced tendency to decompose or agglomerate over time due to nitridization which forms in grain boundaries and inhibits grain boundary movement. AFM studies have shown that grain uniformity is highly improved, as is surface smoothness and thermal stability during rapid thermal annealing at 650xc2x0 C. and 850xc2x0 C. after the refractory metal strip. The addition of the nitride of the diffusion barrier material cover layer produces a large grain structure at the surface which is resistant to deposition at high temperatures and exhibits stability up to 850xc2x0 C. in furnace anneals and 1000xc2x0 C. in rapid thermal anneals.